Gas mileage is the ratio of the number of miles traveled to the number of gallons of gasoline burned. It is related to fuel efficiency and means the same as thermal efficiency or fuel economy. This is the efficiency of converting energy contained in a carrier fuel to kinetic energy or work. This fuel efficiency is rated in the output one gets for a unit amount of fuel input as "miles per gallon" for vehicle-miles. Transportation output can also be measured in terms of passenger-miles or ton-miles (of freight). While the thermal efficiency of petroleum engines has improved in recent decades, this does not necessarily translate into fuel economy of cars, as people in developed countries tend to buy bigger and heavier cars. Non-transportation applications, such as industry, benefit from increased fuel efficiency, especially fossil fuel power plants or industries dealing with combustion, such as ammonia production during the Haber process.

There was an error working with the wiki: Code[14] is one of world's most promising alternative fuel sources. And by developing biodiesel, the world will be less dependent on sources of oil. There is a need for a comprehensive There was an error working with the wiki: Code[15], which includes pursuing policies to make prices reasonable at the pump. Today's gasoline prices and diesel prices are making it harder for families worldwide to meet their budgets. These prices are making it more expensive for farmers to produce their crop, more difficult for businesses to create jobs. Another policy in reducing transportation costs is to better conserve. Inaddition, strategies must be sought to reduce the future damand by countries like India and China for crude oil. This would include develop new fuels like biodiesel and ethanol as alternatives to diesel and gasoline.

Energy efficiency and economy

"Energy efficiency" is similar to fuel efficiency but the input is usually in units of energy such as BTU (British Thermal Units), MJ (MegaJoules), GJ (GigaJoules), kcal (kilo-calories), or kwh (kilowatt-hours). The inverse of "Energy efficiency" is "Energy intensity", or the amount of input energy required for a unit of output such as MJ/passenger-km (of passenger transport), BTU/ton-mile (of freight transport), GJ/tonne (for steel production), BTU/kwh (for electricity generation), or liters/100 km (of vehicle travel). This last term "liters/100km" is also a measure of "fuel economy" where the input is measured by the amount of fuel and the output is measured by the distance travelled. Fuel economy is the amount of fuel required to move a vehicle over a given distance. While the fuel efficiency of petroleum engines has improved markedly in recent decades, this does not necessarily translate into fuel economy of cars, as people in developed countries tend to buy bigger and heavier cars.

{| align="center"

! align = "left"|Fuel type

! align ="right"|&nbsp&nbsp&nbsp&nbspMJ/L

! align ="right"|&nbsp&nbsp&nbsp&nbspMJ/kg

! align ="right"|&nbsp&nbsp&nbsp&nbspThere was an error working with the wiki: Code[8]/There was an error working with the wiki: Code[9]

! align ="right"|&nbsp&nbsp&nbsp&nbspBTU/US gal

! align ="right"|&nbsp&nbsp&nbsp&nbspThere was an error working with the wiki: Code[10]

|-

| There was an error working with the wiki: Code[16]

| align ="right"|29.0&nbsp&nbsp

| align ="right"|45&nbsp&nbsp&nbsp&nbsp&nbsp

| align ="right"|150,000

| align ="right"|125,000

| align ="right"|91&ndash98

|-

| There was an error working with the wiki: Code[11]

| align ="right"|22.16

| align ="right"|34.39

| align ="right"|114,660

| align ="right"|95,475

| align ="right"|115

|-

|There was an error working with the wiki: Code[17]

| align ="right"|19.59

| align ="right"|30.40

| align ="right"|101,360

| align ="right"|84,400

| align ="right"|129

|-

| There was an error working with the wiki: Code[18]

| align ="right"|14.57

| align ="right"|22.61

| align ="right"|75,420

| align ="right"|62,800

| align ="right"|123

|-

| There was an error working with the wiki: Code[12] (10% ethanol + 90% gasoline)

The amount of fuel used per unit distance for example, litres per 100 kilometres (L/100 km). In this case, the lower the value, the more economic a vehicle is (the less fuel it needs to travel a certain distance)

The distance travelled per unit volume of fuel used for example, kilometres per litre (km/L) or miles per gallon (mpg). In this case, the higher the value, the more economic a vehicle is (the more distance it can travel with a certain volume of fuel).

Converting from mpg or km/L to L/100 km (or vice versa) involves the use of the reciprocal function, which is not distributive. Therefore, the average of two fuel economy numbers gives different values if those units are used. If two people calculate the fuel economy average of two groups of cars with different units, the group with better fuel economy may be one or the other.

An interesting example of fuel economy is the popular microcar Smart ForTwo, which can achieve up to 4.0 L/100 km (70.6 mpg) using a turbocharged three-cylinder engine. The Smart is produced by DaimlerChrysler and is currently only sold by one company in the United States. The current record in fuel economy of production cars is held by Volkswagen, with a special production model of the Volkswagen Lupo (the Lupo 3L) that can consume as little as 3 litres per 100 kilometres (78 miles per US gallon or 94 miles per Imperial gallon).

Diesel engines often achieve greater fuel efficiency than petrol (gasoline) engines: 50% of all cars sold in the EU are now diesel vehicles. This can also be attributed to the fact that diesel has 17.6% more energy per unit volume than petrol, and due to economic factors in certain areas, offers more energy for the money.

If one knows the heat value of a fuel, it's trivial to convert from fuel units (such as liters of gasoline) to energy units (such as MJ) and conversely. Except that there are two different heat values for the same fuel and for conversion from electricity to fuel energy, one may need to know how much heat energy from fossil fuel it took to generate the electricity used.

Energy content

The specific energy content of a fuel is the heat energy that is obtained by burning a specific quantity of it (like a gallon, liter, kilogram, etc.). It's sometimes called the "heat of combustion". There exists two different values of specific heat energy for the same batch of fuel. One is the high (or gross) heat of combustion and the other is the low (or net) heat of combustion. The high value is obtained when, after the combustion, the water in the "exhaust" is in liquid form. For the low value, the "exhaust" has all the water in vapor form (steam). Since water vapor gives up heat energy when it changes from vapor to liquid, the high value is larger since it includes the latent heat of vaporization of water. The difference between the high and low values is significant, about 8 or 9%. This accounts for most of the apparent discrepancy in the heat value of gasoline. In the U.S. the high heat values have traditionally been used, but in many other countries, the low heat values are commonly used.

Reducing incured fuel costs

Moving from one place to another takes energy and the following is advice on how to reduce the energy and cost associated with transportation in general, and driving in specific. The first priority is to reduce driving where possible. When driving is necessary, you should try to drive lighter, more conservatively, and in a fuel-efficient vehicle. Driving less will help fueld costs Walk, bike, carpool, or use public transportation whenever possible. If practical for your job, ask your employer to let you telecommute on occasion to save trips to the office. Or, if your work hours are flexible, consider working four 10-hour days a week instead of five eight-hour days, to save a commute each week. Spend some time planning your errands to reduce the number of trips needed. Choose routes that have fewer stops and less congestion. A slightly longer freeway route is usually more efficient than a street route. If possible, avoid driving during rush hour so you aren't wasting fuel in slow-moving traffic.

Driving light will help save on costs. When purchasing a vehicle, choose a smaller, lighter vehicle. You may be able to purchase a small vehicle and borrow or rent a truck or other large vehicle when needed. Keep in mind that smaller vehicles, especially motorcycles, carry higher risks of injury or death in crashes. Don't leave unnecessary items in your vehicle. The clutter may only amount to a few pounds, but hauling it around adds up over time. Consider also that there are around 200 million vehicles in the United States if each of those carries just one pound of excess cargo, it amounts to 100,000 tons. Open windows create significant drag when driving at highway speeds. During warm weather, keep your windows up and use the air conditioner (if needed) when traveling at 45 MPH) or above. When driving at lower speeds, turn the AC off. However, do not continually turn the AC on and off. It functions more efficiently during longer periods of operation. Don't put magnetic decals, flags, antenna decorations, or the like on your vehicle. These create drag.

Driving conservatively will help save on costs. Reduce your speed. The amount of fuel needed to push air out of your vehicle's path increases exponentially with increases in speed. As a result, fuel efficiency drops rapidly. Most vehicles are most fuel efficient between 50 and 55 MPH. However, do not drive so slowly that you impede the flow of traffic, which creates dangerous situations. Accelerate slowly. Getting your vehicle "up to speed" quickly is hard on your vehicle and uses more fuel than a slow start. Also, braking softly farther from your intended stopping point will make your brakes last longer. Use your cruise control. Steady speeds are more efficient. Do not use cruise control in inclement weather. If you are parked for more than a minute, turn your vehicle off. A non-moving vehicle gets zero miles per gallon. However, stopping and starting your vehicle too frequently causes excess wear on your engine. Diesel vehicles should not be turned off frequently as the ignition process uses significant fuel. Also remember to keep your vehicle running efficiently. Keep your tires inflated to the recommended tire pressure. Change spark plugs when recommended by your owner's manual. Check your air filter regularly more frequently if you drive in dusty conditions. To check it, hold it up to the light: if you cannot see light through the filter, replace it. Use regular-grade gasoline unless premium or mid-grade gasoline is specifically recommended by your owner's manual.

Microgravity Fuel efficiency

The energy output derived from fuel occurs during combustion. Ensuring a total, even combustion of fuel, as well as harnessable combustion at the appropriate moments, will have an impact on fuel efficiency. Recent research by the National Aeronautics and Space Administration (NASA) has gained possible insights to increasing fuel efficiency if fuel consumption takes place in microgravity. This probably does not apply to vehicles so much as industry where the benefit from the increased fuel efficiency will outweigh the initial cost of operating in a microgravity environment.

The common distribution of a flame under normal gravity conditions depends on convection, as soot tends to rise to the top of a general flame, such as in a candle in normal gravity conditions, making it yellow. In microgravity or zero gravity, such as an environment in outer space, convection no longer occurs, and the flame becomes spherical, with a tendency to become more blue and more efficient. There are several possible explanations for this difference, of which the most likely one given is that the cause is the hypothesis that the temperature is evenly distributed enough that soot is not formed and complete combustion occurs. Experiments by NASA in microgravity reveal that diffusion flames in microgravity allow more soot to be completely oxidised after they are produced than diffusion flames on Earth, because of a series of mechanisms that behaved differently in microgravity when compared to normal gravity conditions. Premixed flames in microgravity burn at a much slower rate and more efficiently than even a candle on Earth, and last much longer.